pH and pOH: Crash Course Chemistry #30

Summary

This Crash Course Chemistry episode delves into the concept of pH, explaining its significance in measuring the acidity or alkalinity of a solution. The video clarifies the origin of the term 'pH' and its mathematical definition as the negative logarithm of hydrogen ion concentration. It explores the unique properties of water, which can act as both an acid and a base, and introduces the water dissociation constant (Kw). The episode also covers the pH scale, differentiating between strong and weak acids and bases, and introduces pOH. It concludes with a fascinating fact: the sum of pH and pOH in any aqueous solution always equals 14.

Takeaways

• 🧼 pH balance is a concept related to personal grooming products and is essential for maintaining the skin's health.
• 🔍 pH stands for 'power of hydrogen', and it measures the concentration of hydrogen ions in a solution, indicating whether it is acidic or basic.
• 🤔 The 'p' in pH is of uncertain origin, possibly derived from 'power' in various languages, while 'H' stands for hydrogen.
• 💧 Water is unique as it can act as both an acid and a base due to its ability to ionize into hydronium (H3O+) and hydroxide (OH-) ions.
• ⚖️ The pH scale ranges from 0 to 14, with 7 being neutral, below 7 being acidic, and above 7 being basic.
• 📉 pH is the negative logarithm (base 10) of the hydrogen ion concentration, which helps in dealing with very small numbers in chemistry.
• 🔋 The water dissociation constant (Kw) is a special equilibrium constant that equals 1.0 x 10^-14 at 25 degrees Celsius.
• 🔄 The equilibrium concentrations of hydronium and hydroxide ions in pure water are equal and can be calculated using the square root of Kw.
• 📊 pH and pOH are inversely related, and their sum always equals 14, reflecting the product of hydrogen and hydroxide ion concentrations in any aqueous solution.
• 🧪 Strong acids and bases have a significant effect on pH, while weak acids and bases have a lesser effect due to their partial ionization.

Q & A

What does the term 'pH' stand for and why is it written with a lowercase 'p' and a capital 'H'?

The term 'pH' stands for 'power of hydrogen,' referring to the concentration of hydrogen ions in a solution. The 'p' is lowercase because its meaning is not definitively known; it was coined by the Danish chemist Søren Sørensen, who did not explain its origin. Some speculate it might come from a word like 'puissance' or 'pondus,' but it's more likely a convention to differentiate test solutions. The 'H' stands for hydrogen, as hydrogen ions are central to the behavior of acids and bases.

Why is water considered special in the context of pH?

Water is considered special in the context of pH because it can act as both an acid and a base. This is due to its ability to dissociate into hydronium (H3O+) and hydroxide (OH-) ions, which allows it to release or consume hydrogen ions (protons), respectively.

What is the mathematical definition of pH?

The pH of a substance is defined mathematically as the negative of the base 10 logarithm of the concentration of hydrogen ions in the solution. This means that pH is a measure of the relative acidity or basicity of a solution based on the concentration of hydrogen ions present.

What is the significance of the water dissociation constant (Kw) and what is its value?

The water dissociation constant (Kw) is significant because it represents the equilibrium constant for the dissociation of water into hydronium and hydroxide ions. Its value is 1.0 x 10^-14, indicating the product of the concentrations of these ions in pure water at equilibrium.

How does the pH scale relate to the acidity or basicity of a solution?

The pH scale is a measure of the acidity or basicity of a solution, with a pH of 7 being neutral. Values below 7 indicate acidity, with lower numbers being more acidic, while values above 7 indicate basicity, with higher numbers being more basic.

What is the relationship between pH and pOH?

The relationship between pH and pOH is that they are complementary measures in aqueous solutions. The sum of the pH and pOH of a solution is always 14, which is derived from the water dissociation constant (Kw) and the fact that the product of the concentrations of hydrogen ions and hydroxide ions equals Kw.

Why are logarithms used in the calculation of pH?

Logarithms are used in the calculation of pH because they provide a convenient way to express and compare very large or very small numbers, which are common in chemistry when dealing with the concentrations of ions in solutions.

What is the difference between strong and weak acids or bases in terms of their pH?

Strong acids or bases completely or almost completely dissociate in water, resulting in a high concentration of hydrogen ions (for acids) or a low concentration of hydrogen ions (for bases), leading to low or high pH values, respectively. Weak acids or bases only partially dissociate, resulting in lower concentrations of ions and therefore higher or lower pH values compared to strong acids or bases.

How does the addition of an acid or a base affect the pH of water?

Adding an acid to water increases the concentration of hydrogen ions, thus lowering the pH. Conversely, adding a base to water decreases the concentration of hydrogen ions by consuming them, which raises the pH.

What is the practical range of pH values for most substances encountered in everyday life?

The practical range of pH values for most substances encountered in everyday life is typically between 0 and 14, with neutral pH being around 7. However, most substances fall within a narrower range, often between 4 and 9, due to the natural buffering capacity of many systems and the stability of biological and environmental conditions.

Outlines

00:00

🧴 Understanding pH Balance

The paragraph introduces the concept of pH balance, which is crucial in personal grooming products like soaps, shampoos, and deodorants. It explains that pH is related to the equilibrium state of a reversible reaction and is associated with acids and bases. The term 'pH' is discussed, with the 'p' being an undefined symbol and the 'H' standing for hydrogen, which is central to the behavior of acids and bases. The paragraph delves into the mathematical definition of pH as the negative logarithm of the hydrogen ion concentration in a solution, simplifying the concept with examples and explaining the importance of water's ability to act as both an acid and a base.

05:00

💧 The Water Dissociation Constant (Kw)

This section explores the water dissociation constant (Kw), which is a unique equilibrium constant for the self-ionization of water. It describes how water can act as both an acid and a base, forming hydronium (H3O+) and hydroxide (OH-) ions. The equilibrium concentrations of these ions are equal and can be calculated using the formula for Kw, which is 1.0 x 10^-14. The paragraph explains how the pH scale is derived from this constant, with pure water having a neutral pH of 7. It also discusses how changes in hydrogen ion concentration affect the pH, making it a logarithmic scale that simplifies dealing with very small numbers. The concept of strong and weak acids and bases is introduced, along with their effects on pH.

10:01

🔍 Deep Dive into pH and pOH

The final paragraph expands on the understanding of pH by introducing pOH, which is the negative logarithm of the hydroxide ion concentration. It explains the relationship between pH and pOH, demonstrating that their sum always equals 14 in any aqueous solution due to the constant Kw. The paragraph provides an example calculation for the pH and pOH of orange juice, illustrating how these values can be determined and their interdependence. It concludes by emphasizing the practical applications of these concepts in understanding the behavior of acids, bases, and neutral substances in chemistry.

Keywords

💡pH

pH is a measure of the acidity or basicity of a solution and is defined as the negative logarithm of the concentration of hydrogen ions in the solution. In the video, pH is central to understanding how substances interact with water, which can act as both an acid and a base. The pH scale ranges from 0 to 14, with 7 being neutral. The video explains that the pH is calculated using logarithms, which help in dealing with very small numbers, and it's used to determine the strength of acid or base character of a substance.

💡pOH

pOH is the negative logarithm of the concentration of hydroxide ions in a solution. It is introduced in the video as the counterpart to pH, focusing on the concentration of hydroxide ions rather than hydrogen ions. The video explains that the sum of pH and pOH always equals 14, which is a key relationship in aqueous solutions. This concept helps to further understand the balance of acids and bases in chemical reactions.

💡Hydrogen ions

Hydrogen ions, also known as protons, are fundamental to the behavior of acids and bases. The video describes how hydrogen ions are pivotal in determining the pH of a solution. They are released by acids and consumed by bases, and their concentration in water is what defines the pH level. The video also discusses how water can dissociate into hydrogen ions and hydroxide ions, which is crucial for understanding the concept of pH.

💡Water dissociation constant (Kw)

The water dissociation constant, denoted as Kw, is a specific equilibrium constant for the self-ionization of water. In the video, Kw is introduced as a key value in understanding the neutral pH of water, which is 7. The formula for Kw is the product of the concentrations of hydronium and hydroxide ions, and it is always 1.0 x 10^-14 in any aqueous solution at 25 degrees Celsius. This constant is used to calculate the pH and pOH of a solution.

💡Acid

An acid is a substance that donates hydrogen ions (protons) when dissolved in water. The video explains that acids can be strong or weak, depending on the extent to which they dissociate in water. Strong acids, like hydrochloric acid, have a low pH because they release a high concentration of hydrogen ions, while weak acids, like citric acid, have a higher pH because they release fewer hydrogen ions.

💡Base

A base is a substance that accepts hydrogen ions (protons) when dissolved in water. The video describes how bases can also be classified as strong or weak. Strong bases, like sodium hydroxide, have a high pH because they consume a large number of hydrogen ions, while weak bases, like baking soda, have a lower pH because they consume fewer hydrogen ions.

💡Litmus

Litmus is a natural pH indicator used in the video to demonstrate the concept of pH visually. It changes color depending on the pH of the solution it is in, turning pink in acidic conditions, blue in basic conditions, and a light purple in neutral conditions. This property of litmus is used to illustrate the pH scale and the concept of acidity and basicity.

💡Logarithms

Logarithms are mathematical operations that are used in the video to explain the calculation of pH. The video simplifies the concept by explaining that the pH is the negative logarithm of the hydrogen ion concentration. Logarithms are used because they allow chemists to work with very small numbers that are common in solutions, making the calculations more manageable.

💡Equilibrium

Equilibrium in the context of the video refers to a state in which the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of reactants and products. The video discusses how water can exist in a state of equilibrium, where it can both dissociate into hydrogen and hydroxide ions and recombine to form water molecules.

💡Reversible reaction

A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions under the same conditions. In the video, the dissociation of water into hydrogen and hydroxide ions is described as a reversible reaction, which is constantly occurring, but the concentrations of ions remain constant due to the equilibrium state.

Highlights

The concept of pH balance is related to the equilibrium state of a reversible reaction.

pH scale is associated with acids and bases, but the lowercase 'p' and uppercase 'H' have an unclear origin.

The 'H' in pH stands for hydrogen, which is central to the behavior of acids and bases.

pH is defined as the negative of the base 10 logarithm of the concentration of hydrogen ions in a solution.

Water can act as both an acid and a base due to its ability to release or consume hydrogen ions.

The dissociation of water into hydronium and hydroxide ions is a reversible reaction.

The water dissociation constant (Kw) is a special equilibrium constant for the dissociation of water.

In pure water, the concentrations of hydronium and hydroxide ions are equal at equilibrium.

The pH of pure water is 7, which is considered neutral.

The pH scale ranges from 0 to 14, with values below 7 indicating acids and above 7 indicating bases.

Strong acids have low pH values due to their high hydrogen ion concentration.

Weak acids have higher pH values because they release fewer hydrogen ions.

Strong bases have high pH values because they consume hydrogen ions, lowering their concentration.

Weak bases have pH values in the 8-11 range due to their limited consumption of hydrogen ions.

The pH and pOH of a solution are interconnected, with their sum always equaling 14.

Litmus paper is a pH indicator that changes color based on the pH of a solution.

The episode covers the calculation of pH and pOH using logarithms and their practical applications.

Transcripts

This may come as a bit of a shock to you, but I'm not super into personal grooming.

Like I understand soap and shampoo.

But there's all this other stuff now, and I keep seeing references to pH balance everywhere.

pH balanced soaps, and shampoos, and deodorant, and makeup abound in supermarkets and drugstores.

And I've even seen pH balanced water?!

We've talked a lot about balance over the last couple of weeks,

and pH balance is related to the equilibrium state of a reversible reaction.

You're probably also familiar with the pH scale, and you know that it has to do with acids and bases.

But what is pH exactly?

And why is it weirdly written with a lowercase 'p' and a capital 'H'?

And also, what about pH's alter ego pOH?

The capitalization thing is probably the easiest to answer, 'cause there is no answer.

No one really knows what the 'p' means.

The Danish chemist who came up with the term,

a guy with the absolutely amazing name, Søren Sørensen, never explained his reasoning.

Some people think that it comes from some form of the word power,

whether it's puissance in French, or maybe Latin, pondus.

But it probably just came from a common habit chemist have of differentiating a test solution,

labeled 'p', from a reference solution, called 'q'.

But thinking of the 'p' as standing for power, does help us remember the meaning more easily.

The 'H' part is even easier, it stands for hydrogen.

Because hydrogen ions, or protons, are pivotal to the behavior of acids and bases, which is what pH describes.

So you can think of pH as basically the power of hydrogen in a solution.

The strength of the acid or base character of a substance.

And it all revolves around one very important point of focus, our old friend water.

[Theme Music]

If you've been watching Crash Course Chemistry from the beginning you've gotten the message

that water is special in more ways than I can list, and pH is just one more of those ways.

We normally think of water as a perfectly neutral substance, neither acidic nor basic, and that's true.

But, as I've mentioned before, water can also function as an acid --

releasing hydrogen ions, also known as protons, and as a base -- consuming them.

How on Earth is that possible?

In order to explain, we first have to understand what the pH of a substance really tells us.

While chemically we say that pH represents the power of hydrogen in a solution, it's mathematically defined as:

"the negative of the base 10 logarithm of the concentration of hydrogen ions in solution."

OK, so now that you're terrified, I'm here to help.

So, yeah, logarithms can seem a little bit scary at first, but the ones that we're using here are super easy.

And bonus, once you get familiar with them here, it'll be that much easier to understand them in math class.

So now that we got the scary mathematical definition, let's do the simplest mathematical definition.

At any given moment, there will be a certain number of hydrogen ions in solution -- a very small number --

the concentration will be a number like one times 10 to the negative fifty moles per liter.

That negative 5, is your base 10 logarithm.

Take the negative of that, and you get the pH. 5.

Now let's get a bit more in to the weeds.

The logarithm, or log, of a number is the exponent to which another number, called the base,

must be raised to produce the target number.

So for base 10 logs, the base is 10.

They're what we use most in chemistry, and they're really easy to understand,

and also what we base scientific notation on.

So as an example, the base 10 logarithm of one hundred is 2.

Because 10 raised to the power of 2, or 10 squared, equals one hundred.

Base 10 logs are so common that we often leave the subscript 10 off when we write it.

Like if your calculator has a 'log' button, that's just for base 10 logs.

So what in the name of Søren Sørensen does this have to do with face melting acids?

Well I'm getting to that, and it all starts with waters crazy potential to act as both an acid and a base.

Random changes in the tiny electrical fields around the atoms in water

occasionally cause the molecules to break apart.

Specifically a hydrogen ion, or proton, will break off from one molecule and attach itself to another one,

forming a hydronium ion, H3O+, and a hydroxide ion, OH-.

This is why water can act as both an acid and a base.

It's molecules can both release and accept protons.

In this case, it's only interacting with itself.

But water can interact in the same way with other acids and bases.

Some times you'll see the hydronium ion written as a simple hydrogen ion, H+,

allowing the reaction to be written with only one water molecule.

It's not technically accurate, but it's close enough to reality that it can be used to simplify things.

So when we say that the pH is the negative log of the hydrogen ion concentration...

yeah, we actually mean hydronium ion concentration.

Just another thing that early scientists got a little wrong and now we have to live with.

Anyway, this dissociation of water is a reversible reaction.

And in fact, the ions always reform in to water within a tiny fraction of a second.

But it's happening all the time constantly.

In your bottled water, in the water inside your cells, and in the ocean. Always.

However, at any given instant only a tiny number of molecules are dissociated ions.

In fact, the exact number of these molecules is well known to chemists.

It's the equilibrium constant for this reaction.

And because it's such a special reaction, it has it's own name -- the water dissociation constant, or Kw.

Kw is equal to one point zero times 10 to the negative fourteenth.

The formula for Kw is set up like any equilibrium constant,

concentrations of products over concentrations of reactants,

all raised to the exponents based on the coefficients of the balance reaction.

There is however one difference.

Because the ions represent such a tiny proportion of the total mass, the water itself is essentially pure.

And pure substances, because they don't have concentrations, aren't included in equilibrium calculations.

So the formula for Kw becomes simply the hydronium ion concentration times the hydroxide concentration.

According to the balanced equation for the dissociation of water,

hydronium and hydroxide are formed at a 1:1 ratio,

so their equilibrium concentrations must be equal.

That means if we call the concentration of H3O+, for example 'x',

then the concentration of OH- must equal 'x' as well.

So the formula for the dissociation constant 1.0 x 10^-14 simplifies even further to x times x, or x squared.

Suddenly, it's crazy easy.

The equilibrium concentration of each ion is just the square root of 1.0 x 10^-14.

Touch one key on the 'ol calculator, and hello both concentrations equal

1.0 x 10^-7 moles per liter in equilibrium.

The pH then, is simply the negative log of that, which is 7.

This my friends, is the basis of the pH scale.

Water is neutral, so 7 is the center of the scale.

And I can prove it too.

This is a strip of paper that's been infused with a chemical called litmus.

Litmus is a pH indicator, a chemical that turns different colors at different pHs.

There are many different indicators, with many different colors, but we'll talk more about those next week.

For now, just know that litmus paper turns pink in acids, blue in bases, and a sorta light purple when it's neutral.

But one thing you need to remember about the pH scale,

because pH is calculated from a negative logarithm, it turns everything backward.

When the hydrogen ion concentration goes up, the pH gets lower.

For instance, if a little acid, such as vinegar, were added to the water,

the concentration of hydronium ion might rise to say, 1.0 x 10^-4 moles per liter.

Which is a thousand times more than before.

That concentration would push the pH down to 4.

On the other hand, a base, such as ammonia,

would consume a lot of hydrogen ions if it were added to the water.

If the hydrogen ion concentration drops to 1.0 x 10^-11,

a thousandth of the equilibrium concentration, the pH would be eleven.

As you can see, the logs turn out to be a mathematical shorthand,

that saves us from dealing with very huge or very tiny numbers.

The pH scale then is normally written from 0 to 14.

With numbers below 7 representing acids, and numbers above 7 representing bases.

It could also below zero or above 14, but that only happens in super extreme cases,

that you are very unlikely to encounter.

At least I hope.

As with like hydrochloric or nitric acid, which ionize strongly, sometimes even completely.

Thus releasing a lot of protons, are called strong acids.

Because they raise the hydrogen concentration a lot, they also generally have very low pHs.

Weak acids, like citric acid, dissociation incompletely,

releasing much smaller amounts of hydrogen ions, and therefore they usually have higher pHs.

Generally considered in like the 4 to 6 range.

Strong bases meanwhile, like sodium hydroxide,

consume large amounts of hydrogen ions leaving the concentration very low, so they 10d to have very high pHs.

Weak bases, like sodium bicarbonate (baking soda), consume much less, and generally have pHs in the 8-11 range.

Neutral pH is technically just 7.0, but in a more practical sense, it's usually considered to be between 6 and 8.

So if pH is based on the concentration of hydrogen, that is hydronium ions,

what about the concentration of hydroxide ions?

Just as we can calculate the pH of a substance from it's hydrogen ion concentration, we can calculate the pOH.

The negative log of the hydroxide concentration.

This is easy because Kw never changes.

Although the concentrations of hydrogen and hydroxide are only equal in pure water, or perfectly neutral solutions,

the product of the 2 concentrations always equals 1.0 x 10^-14 in any aqueous solution.

So like orange juice, which is really just an aqueous solution of sugar and citric acid and a few other things,

say the hydrogen concentration in your OJ is 3.2 x 10^-4 moles per liter.

Just for the fun of it, let's go ahead and calculate what the pH is at that point, which turns out to be 3.5.

But we can also use the Kw and the hydronium ion concentration

to do a very simple division problem and find the hydroxide concentration.

It works out to 3.1 x 10^-11 moles per liter.

Once we have the concentration we can take another step,

we can find the pOH of the solution which is similar to the pH, simply the negative log of the OH concentration.

The pOH in this case is 10.5.

And now for a tip that's just more awesome and cooler than an ice cream corn dog,

the sum of the pH and the pOH is always 14.

In the example we just did, the pH was 5.4 and the pOH was 8.6, and yeah you add those together: 14! Surprise!

OK, maybe that's only cool to me.

But that's never stopped me before, I love this stuff!

And next week, I hope to really bend your mind by showing you how to make the pH of a solution hold steady,

even if you dump a strong acid or base in it.

In the meantime, thank you for watching this episode of Crash Course Chemistry.

If you paid attention, you learned

how pure water ionizes to form hydronium and hydroxide ions in reversible reactions.

And you learned about the equilibrium constant for that reaction,

which has a special name: the water dissociation constant.

You learned some examples of acids and bases and neutral substances,

as well as why some acids and bases are called strong and others are called weak.

You learned about logarithms and how you can use them to calculate the pH of a substance.

And a little bit about pOH, which can be calculated with logarithms, also with subtraction.

And finally, you learned about some cool mathematical connections between pH and pOH.

This episode was written by Edi González and edited by Blake de Pastino.

The chemistry consultant was Dr. Heiko Langner.

It was filmed, edited, and directed by Nicholas Jenkins. The script supervisor was Katherine Green.

Michael Aranda is our sound designer, and our graphics team is Thought Café.